Many homeowners are replacing conventional doorbell with a smart doorbell. For an easy do-it-yourself install, a desirable feature for smart doorbells is to use the door chime and the pre-existing doorbell wiring of the household as the power supply. Typically, conventional doorbells use a low voltage transformer of to 24 VAC. The smart doorbell may then be powered by the existing power supply and wiring and the existing chime(s) may serve as an annunciator(s) for visitors.
Door chimes are typically connected in series with the transformer, which results in the chime impedance limiting the power available to the smart doorbell. If the smart doorbell draws even a small portion of the current normally required to operate the door chime, a mechanical solenoid chime may respond with buzzing noises as the solenoid moves back and forth against the spring at the line frequency (60 Hz or 50 Hz) or at a harmonic or subharmonic of the line frequency. An electronic chime may also be activated or make undesired sounds at much lower currents than a smart doorbell requires for operation.
One typical solution to the problem of supplying the necessary current for operating a smart doorbell is to use a normally closed relay connected across the door chime that bypasses the door chime. Bypassing the door chime can supply more current to the smart doorbell when the door chime is not sounding. When the smart doorbell needs to sound the door chime, a switch is closed in the smart doorbell that increases the current flowing through the chime bypass. The increase in current is detected by the chime bypass and the relay is opened, typically for a fixed length of time. Opening the relay allows the chime to sound similar to closing a switch for a conventional mechanical doorbell.
There are several problems with existing normally closed relay chime bypass designs. One problem is that a fixed current threshold is typically detected to open the relay. Since current flowing can vary widely, having the fixed current threshold results in difficulty for tolerating a wide range of transformer voltages, different numbers of connected chimes, different chime mechanisms and/or electronic circuitry.
Another problem occurs due to the time delay required to operate typical relays, including solid state relays. Because the relay is normally closed, a surge current flows when the doorbell switch is first closed until the relay opens. The surge current can harm either the relay and/or the doorbell switch if not limited with protection circuitry, fuses, or other means.
Another problem occurs with electronic chimes that require a diode across the doorbell switch to provide standby power to the chime electronics. For the electronic chimes, a normally closed relay will not work, as the half-wave power required to “precharge” the chime electronics in the non-sounding state will be shorted out. Shorting out the chime electronics causes a distorted and/or delayed chime sound as the chime power is suddenly turned on fully.
Also, there are a limited number of manufacturers of otherwise little-used depletion mode field effect transistors (FETs) and solid state relays, so they are expensive.
It would be desirable to implement chime control companion and communication.